Process of purifying sodium silicate solutions



Patented Dec. 15, 1931 EDWARD A. TAYLOR, 0E CLEVELAND, AND woErHINGro 'ri GEAoE ,"oE:wILLouGi1iir,f

OHIO, ASSIGNORS, BY MESNE AssIGNMEN'rs; 'ro'rrrE' GBAssELLI o ntly:IoA'Lf fcoiir PM, or CLEVELAND, OHIO, A CORPOR TION OEDEL WARE PROCESS OF PURIFYING SODIUM SILIQATESOLUTIONS No Drawing.

The present invention relates to purification of sodium silicate solutions and to processes of precipitating unfilterable, finely divided material contained in such solutions, to render the same amenable to ordinary technical filtration operations.

Solid sodium silicate glass contains regularly small amounts of impurities, such as iron, aluminum, calcium and magnesium compounds, probably in the form of silicates which originate partly in the quartz, sand, and other raw materials used or which are obtained from the furnace walls by the action of the strongly alkaline silicate melt. Part of these impurities will dissolve together with the sodium silicate when a very concentrated solution is prepared, but some of these remain insoluble and form a suspension of more or less colloidal particles which either pass through filter cloth or form a slimy precipitate thereon, which rapidly clogs the pores of the filters and prevents further passage of the viscous silicate solution. D1lution of such turbid solutions below the concentration of commercial silicate solutions proportions of aluminum and iron, and particularly when they contain considerably more of these than of calcium salts.

We have now found that by precipitating an insoluble material in the silicate solution, the physical structure of the more or less colloidal impurities contained therein is modified in such a manner that they will settle out and are readily retained on a filtering medium without unreasonable clogging of the filter-cloth, and that the solutions obtained by such a treatment and subsequent Application filed August 3o, 1928." ,S eria1 Nq. ,so3 ,11 s.

amnion are perfectly am a d are that,

and remain in; ,this, condition evenifftheir,

concentration ischanged by dilution or evapo r'ation."

The formation of a precipitate Cate solution can beeffected byaddiri ",there to; twomateria g which will reactvtto ormjan insol'ubleprecipitate, as, for instance, v sulfate and milk of llme. )As a more conveni ent operation, wepre'fe'r to add the precip itatinto thev dissolver .wliich f can. be a rotating autoclave charged with. the ingredientsyand inla' physical statewhich 4 allows of easyfsetf; A

tli'n'g and filtration.

Amon st various p ri atins s l s we found the most effective, to epalciu l .salts. Additions of'quick lime-on hydrated lime to the, silicate'dissolven charge result in the formation of insoluble calcium silicate which so modifies the/iron and'aluminum' impurities that they willcompletelyflsettle out in a, few days,'leaving asupernatenaeiear liquorjwhich isTjfilter-pressed without dif-,'.: ficulty; The amount of lime required is j usually ,a' fraction: of 1 70 of the weightof the'silicatefl A quicker settling and coarser V is formed when sodium carbonate isfa dded together with the-lime and the silicate solu 7 tion filters withg'reat 'facilitythrough filterpresses "equippedwith ordinary filter-cloth. The action of these calcium salts is appar-- ently entirely mechanical and serves to agglomerate and distribute the slimy iron and aluminum compound throughout the whole mass, forming a coarser grained non-porous filtercake, through which the viscous silicate solution passes readily. I The amount of hydrated lime and sodium carbonate needed to produce the best filterable silicate solution varies to a certain extent with the composition of the silicate and the amount of impurities contained in the glass,- but will be usually below 1% of the weight a", of the glass. 020 lbs. hydrated lime and 1 ing reagentstogetherwith the silicate glassj 0.295 sodium carbonate for each 100 lbs. of

glass were found entirely satisfactory with a silicate in which the ratio of SiO to Na O was 3:1 and which was made from quartz sand containing 0.64% iron and aluminum oxide, 0.028%Ca0 and 0.029'% MgO. 'Dissolvedto a 40 B. solution, this glass, with the above additions, settled completely in abouttwo days, and when filtered, even be-\ fore. settling,v allowed the filtration of about; 100,000 lbs. solution in less than 24; hours. through a standard 36 plate and frame filter-press, producing a perfectly; clear water- 7 white silicate. Filtration without the addition of the lime and carbonate produces-a slightly turbid solution, and even then, it is. necessaryto clean out the press severaltimes as the slimy precipitates quickly clog the filter-press and entirely prevent the passage 20? of the solution.

We claim:

1. In a process ofpurifying a sodium silicate solution containing suspended insoluble impurities the steps of forming a precipitate of-"an alkaline earth metal salt within said 'solution and removing the combined insolubleand precipitate from said solution.

2; The process of claim. 1 whenever the precipitate of alkaline earth metal salt is H formed-{during dissolution of the sodium sili-v cateinWater.

' 3; Ina process ofproducinga purifiedso lution of sodium silicate which'comprises;

dissolving water glass by the action o ffwater and steam thereon, the steps'of adding lime and a soluble carbonate'to'the chargegof Water removing the precipitate from thesolution vglass and water, and removing theprecipitate frornthe solution formed;

, 4C Ina process of producing 'a'purifi'ed solution of sodium silicate which-comprises dis solving'water glass by the action of water and steam thereon, the-steps of adding less than 11% of. hydrated lime and sodium carbonate, figured on the weight of the silicate, to the charge of water glass and water and,

firmed.

" intestlmony h w a Sigma es-i I a EDWARD A. TAYLOR.

' V WOBTHINGTON T. GRAQE. 

